Wow, I'm ashamed to know this, but... As I remember it, when Geico first started airing the gecko commercials a few years back, he did have ad "Australian" type accent. Which I always assumed was to appeal to fans of the then-popular Steve-Irwin-beats-the-shit-out-of-wildlife shows. They soon changed his accent to a generic BBC one. Then, more recently, his accent was changed to the current Cockn... er, "Estuary" accent.

Wikipedia says that the accent was always British...

"In the first commercial, where people kept calling the gecko as a wrong number for GEICO, the gecko was given a high-class British accent because it would be unexpected, according to The Martin Agency's Steve Bassett. In current commercials the gecko's accent is more working-class, to further "humanize" him and make him "more accessible and someone you enjoy listening to, some bloke you want to get to know."

...but I do seem to remember the early commercials with the "Aussie" accent. Whatever.

---Lepus Rex